The woman is suspected of killing the child when the youngster was in her care, officials said.

Videos posted on Russian news websites show a woman dressed all in black, holding the severed head and shouting "I am a terrorist" in Russian, although most of what she says is incomprehensible.

In some of the videos the woman is tackled by men who appear to be police.

The Investigative Committee released a statement saying a woman was arrested on suspicion of killing a child aged three or four in an apartment near the metro station in north-western Moscow and then setting the apartment on fire.

The woman, a 38-year-old native of one of the former Soviet republics in Central Asia, was believed to be the nanny and appeared to be mentally unstable, the statement said.

The investigators' report did not link the two incidents. However, an official in the Investigative Committee confirmed that it was the same woman.

The official report said the child's body found in the apartment showed signs of a violent death. Russian news reports, citing law enforcement sources, said the child had been decapitated.

Investigators said they were trying to determine the suspect's motive. She appeared to have waited until the parents and an older child had left the apartment before killing the younger child, the statement said.